Congenital fourth nerve palsy is a condition present at birth characterized by a vertical misalignment of the eyes due to a weakness or paralysis of the superior oblique muscle. The fourth cranial nerve innervates the superior oblique muscle for each eye. The superior oblique muscle is one of the six extraocular muscles that allow movement of eye. Specifically, the superior oblique muscle is used for downgaze and abduction (looking away from the nose). In addition, the superior oblique produces an intorsion(rotation such that the top of the eye rolls toward the nose). When this muscle’s function is diminished due to a fourth nerve palsy, the affected eye will deviate upward resulting in hypertropia, or, vertical strabismus with lesser components of horizontal and oblique diplopia (double vision).

The cause of congenital fourth nerve palsy is unclear in most cases. It may be neurogenic in origin, due to an abnormality of the fourth cranial nerve or dysgenesis of its nucleus, or it may result from absence or dysfunction (e.g., abnormal laxity) of the superior oblique tendon. Though usually unilateral, congenital fourth nerve palsies can also occur bilaterally. Bilateral congenital fourth nerve palsy may be unmasked only after corrective surgery of one eye for what was thought to be a unilateral palsy.

Though the condition is congenital the symptoms of a congenital fourth cranial nerve palsy do not begin until many years later. Indeed most patients only notice the diplopia in middle or late adulthood. In young children, it may manifest with abnormal head posture. The characteristic head tilt is usually away from the affected side to reduce the second image. Old photographs may reveal the presence of a consistent head tilt (torticollis) from an early age. Other compensatory measures for congenital fourth nerve palsy are development of large vertical fusional amplitudes and lack of subjective symptoms of torsion, even in the presence of great ocular rotation. Congenital fourth nerve palsy may remain undetected until adulthood, when it becomes noticeable to the patient for the first time as intermittent diplopia due to decompensated ability to overcome the vertical deviation. This may occur from specific causes of fatigue (stress, fever, other illnesses) or simply the effects of old age. Alternatively, the patient may develop symptoms in adulthood in the form of neck pain, after years of head tilting. Some patients may have apparent facial asymmetry. Diplopia from congenital fourth nerve palsy has been reported to manifest transiently during pregnancy in some cases. Congenital fourth nerve palsy may also become evident following cataract surgery once binocular vision is restored after a long period of progressively monocular vision and accompanying decompensation.

Symptomatic congenital fourth cranial nerve palsy can be treated with strabismus surgery, where muscle attachment sites on the globe are modified to realign the eyes. Prism lenses set to make minor optical changes in the vertical alignment may be prescribed instead of or after surgery to fine-tune the correction.